Plants consume large quantities of CO2 based on photosynthesis, in which CO2 and H2O are converted to carbohydrates using chlorophyll under sunlight. However, the planet’s largest forest, the Amazon, which greatly contributes to the removal of atmospheric CO2, is continually shrinking because of commercial development and serious fires. CO2 also dissolves in the oceans to form H2CO3, HCO3− and CO32−, and there is approximately 50 times as much carbon dissolved in the oceans as exists in the atmosphere.1) In contrast, all living organisms produce CO2 during respiration, such that the rates of CO2 consumption and production were balanced before human activities produced huge amounts of CO2. Certain CO2 derivatives are used industrially2) and in medicine.3) As an example, our own group recently found that NaHCO3 and Na2CO3 accelerate glucose consumption in cultured cells.4,5) These materials have also been shown to improve serum glucose levels in diabetes mellitus patients.6) However, the rate of usage of CO2 compounds in such applications is obviously much smaller than the rate of CO2 production.
CO2 can be captured from the ambient air or from flue gas via several techniques, including absorption,7) adsorption8,9) and membrane gas separation.9,10) Absorption with amines is currently the dominant technology, while membrane and adsorption processes are still in the developmental stages with the construction of primary pilot plants anticipated in the near future. However, to the best of our knowledge, these methods alone cannot achieve the necessary worldwide reductions in atmospheric CO2.
In the present work, CO2 was bubbled through an initially clear solution containing 0.05 N NaOH and 0.05 M CaCl2 to form an immediate white precipitate (Fig. 1). In other trials, varying the NaOH concentration between 0 and 0.5 N in the presence of 0.05 M CaCl2 was found to generate a white precipitate above 0.2 N NaOH even in the absence of CO2. Because this precipitate resulted from the formation of Ca(OH)2, the potential for CO2 incorporation in the form of CaCO3 was minimal under these conditions. In contrast, solutions having lower NaOH concentrations (from 0.05 to 0.1 N NaOH) together with 0.05 M CaCl2 remained clear, while the addition of CO2 bubbles produced a white precipitate (Fig. 2a). Under these conditions, CaCO3 precipitation occurred in the presence of CaCl2, meaning that high NaOH concentrations were reduced by the formation of a Ca(OH)2 precipitate. However, prolonged bubbling with CO2 decomposed the CaCO3 precipitates to form Ca(HCO3)2, which is water soluble. As the concentration of CaCl2 was changed from 0 to 0.5 M, the amount of white precipitate was found to plateau at 0.05 M CaCl2 (Fig. 2b).
The CO2 concentration in a 2-L bottle made of poly(ethylene terephthalate) (PET) was monitored to determine whether a solution containing 0.05 N NaOH and 0.05 M CaCl2 reduced the level of CO2. These trials showed that the CO2 reduction was clearly correlated with the time span over which the solution remained in the bottle and in contact with the internal atmosphere (Fig. 3a). Approximately 60% and 80% of the initial CO2 was removed after 15- and 60-min treatments, respectively. After allowing the plastic bottle to sit overnight, the CO2 in the bottle was completely removed. Thus, even extremely low CO2 concentrations could be efficiently captured and fixed by a solution containing 0.05 N NaOH and 0.05 M CaCl2. Laying the plastic bottle on its side increased the surface area of the solution and thus increased the CO2 removal rate (Fig. 3b).
At a high CO2 concentration of approximately 15%, the addition of 50 ml of a solution containing 0.05 N NaOH and 0.05 M CaCl2 followed by vigorous shaking of the 2-L bottle for 30 s by hand reduced the CO2 concentration to 10% (Fig. 3c). A further slight reduction of the CO2 concentration was obtained by subsequently allowing the bottle to stand. The addition of 50 ml of a fresh solution also resulted in an additional slight reduction and a further addition of fresh solution after 24 h again reduced the CO2 concentration (Fig. 3c). This slow reduction of the CO2 level after the initial rapid removal is attributed to the presence of insufficient quantities of NaOH and CaCl2. The pH of the solution after 24 h and following the third addition was 6.5, while that of the initial fresh solution was 12.19. These results indicate that the NaOH in the solution was completely consumed.
In the above trials, a solution containing low concentrations of NaOH and CaCl2 was used in a one step process. When using high NaOH concentrations (above 0.2 N), the CO2 should first be treated solely with NaOH to prevent the formation of Ca(OH)2. This produces a solution of NaHCO3 and Na2CO3 to which CaCl2 can be added after reducing the NaOH concentration to less than 0.1 N. The latter method is based on two steps, and allows the use of high concentrations of NaOH and CaCl2.
Because increasing the surface area of the highly concentrated NaOH solution is also important to ensuring efficient absorption of CO2, the generation of a fog can be beneficial. The formation of a fog greatly increases the liquid surface area, resulting in more rapid CO2 removal in the plastic bottle (Fig. 4a). In experiments using a chimney model, when the chimney contained high CO2 concentrations, the amounts of NaOH and CaCl2 in the solution were insufficient to react with all the CO2 at a gas flow rate of approximately 110 cm3/s (Fig. 4b). Thus, the solution was only able to capture a relatively small amount of the CO2 in the chimney model..
The area over which the reagent solution interacted with CO2 could also be increased by bubbling the test gases through a porous stone medium. In these trials, a poly(vinyl chloride) pipe (40 mm in diameter and 50 cm high) was partially filled with 250 mL each of aqueous solutions containing 0.1 N NaOH and 0.1 M CaCl2. Following this, the test gas was bubbled upwards through the solution at a flow rate of approximately 20 ml/s after passing through the porous stone at the bottom of the pipe. Under these conditions, the CO2 contained in the air was completely absorbed by the solution (Fig. 5a). In trials using this same apparatus with a very high CO2 concentration, the level was reduced from an initial value of 10% to 2.5% (Fig. 5b). These data indicate that this concept could be employed to reduce high CO2 levels in the exhaust streams from industrial operations such as thermal power plants and incinerators.
One means of producing NaOH on an industrial scale is the electrolysis of an aqueous NaCl solution. The products of this newly developed CO2 fixation system based on NaOH and CaCl2 are CaCO3 and NaCl, and this NaCl could therefore be subsequently converted to NaOH, H2 and Cl2 via an electrolytic process. Thus, CO2 could be captured using this system while simultaneously producing H2 and Cl2 (Fig. 6). In addition, this process could potentially be integrated with existing generator systems based on atomic, thermal, solar, wind, hydro or wave power, and natural seawater could be used instead of an artificial NaCl solution in the electrolysis process.
Recently, plastic waste has been shown to be a significant environmental pollutant, and micro-plastics have been found to affect marine organisms. A small portion of the plastics that are used daily in human activities are recycled, while the remainder is simply treated as waste. Many of these materials could be incinerated but instead are typically sent to landfills. However, if a simple method of fixing CO2 becomes available, this waste could be readily disposed of by burning without any environmental concerns and with the potential to generate energy. At present, chemical absorption using organic amines is typically employed to capture CO2 emitted from thermal power plants, but liberating CO2 from these complexes requires heat treatment that induces degradation. Because this treatment itself produces CO2, a new method that fixes CO2 would be highly beneficial. The present method employing inorganic compounds generates a stable product, based on the neutralization of NaOH along with the formation of CaCO3 and NaCl, both of which are harmless, stable natural compounds.
This technique is applicable to thermal power plants, chemical plants, large ships, combustion operations, incinerators and automobiles. Using this process, atmospheric CO2 can be spontaneously fixed based on a simple apparatus at various locations to generate CaCO3. This newly developed energy recycling system has minimal environmental impact and is completely sustainable, and so is expected to provide a means of reducing atmospheric CO2 levels so as to mitigate climate change.